Marine Sediment Characterized by Ocean‐Bottom Fiber‐Optic Seismology

The Sanriku ocean‐bottom seismometer system uses an optical fiber cable to guarantee real‐time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high‐resolution 2‐D shear‐wave velocity profile by inverting multimode dispersion curves extracted from frequency‐wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean‐bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost‐effective ocean‐bottom imaging in the future.Plain Language SummaryDistributed Acoustic Sensing (DAS) is a relatively new measurement method that has the potential to convert existing fiber optic communication infrastructure into arrays of thousands of seismic sensors. In this research, we connected a DAS to a cable that was originally installed at the bottom of the ocean to sustain a seismic and tsunami observatory in the Sanriku Region. We show that this new type of measurement can provide reliable information to image and explore the shallow subsurface under this fiber cable. This is the first time such analysis is performed in an oceanic environment, and our methods could be readily exportable to other fiber‐optic cables that are the backbones of our modern telecommunication.Key PointsOcean‐bottom Distributed Acoustic Sensing is used to image shallow VS structureRayleigh wave phase velocity dispersion curves are extracted from frequency‐wave number analysisReflection image is obtained from autocorrelations of ambient seismic fieldPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/3/grl61098_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/2/grl61098.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/1/grl61098-sup-0001-2020GL088360-Text_SI-S01.pd

Improvement of Battery Assembly for Ocean Bottom Seismometers and Outsourcing of Assembly

A plastic holder was developed for primary lithium batteries, which are assembled at the Earthquake Research Institute, as a power source for an ocean bottom seismometer (OBS). There are three different sizes of plastic holders that can hold 2-4 cells. The holders have a simple structure that consists of two parts made of polyvinyl chloride, and they do not require special tools for assembly. Therefore, high quality assembly has been achieved and the time for assembly of a battery has been significantly reduced. Weight is always a problem for seafloor instruments to secure positive buoyancy for recovery. Therefore, the holders must be made as light as possible. The holders are also made of translucent plastic for visual inspection after assembly. A total of more than 10,000 holders have been produced and were shipped by Sept. 2019. The development of these holders has enabled us to outsource assembly of the batteries and achieve efficient battery production. The outsourced assembly of batteries for OBSs is now established as an effective means of assembly.＜論説

Precipitates on Beach Electrical Ground of Optical-cable-type Ocean-bottom Seismometer and Tsunami Meter System off the Sanriku Coast

We found precipitates on the beach electrical ground (negative electrode) of the optical-cable-type ocean-bottom seismometer and tsunami meter system off the Sanriku coast (Sanriku system). These precipitates formed from 2014 to 2018. The DC voltage that feeds the Sanriku system began to increase and fluctuate in June 2017. This fluctuation suggests instability in the Sanriku system. To investigate the relationship between the precipitate growth and supply voltage fluctuation, we sampled and analyzed the precipitates in September 2018. The main components of the precipitates were calcium carbonate (CaCO3) and dolomite (CaMg(CO3)2), which are abundant in seawater. The metallic positive ions in seawater were drawn to the negative electrode, precipitating on the beach electrical ground and increasing its electrical resistance. This change in electrical resistance caused the observed DC voltage fluctuation. The beach electrical ground should be regularly cleaned to maintain the stability of the Sanriku system.＜論説

昭和南海地震震源域-四国沖～紀伊半島沖-の構造変化

BE12-54講演要旨 / ブルーアース2012（2012年3月22日～23日, 東京海洋大学品川キャンパス

Structural variation from off Shikoku to the Kii Peninsula related to various earhthquake phenomena

南海トラフで繰り返し発生する巨大地震の中には、東海・東南海・南海地震が連動して発生する超巨大地震のケースがあることが指摘されている。このような超巨大地震について、どのような場合に連動し、あるいは非連動性巨大地震となるのかを明らかにすることが必要である。連動型地震の滑り域の範囲がどこまで広がるかを見積もることが重要な課題であり、そのためには南海トラフから沈み込むフィリピン海プレートの形状およびプレート境界周辺の構造、地震活動に関する詳細かつ高精度な情報が必要である。また、南海地震単独で発生した場合についても、深部および浅部低周波地震発生域まで含めた正確な破壊の広がりの把握、複雑な破壊分布の原因を明らかにするために南海地震破壊域とその縁辺での地殻構造や地震活動は重要な情報となる。平成20年度は日向灘、平成21年度は四国沖で調査を実施し、平成22年度は調査海域を東方の紀伊半島沖まで拡大し、沈み込み帯の地殻構造、巨大地震の発生、地震活動の相互関係の解明を目的として、南海・地震破壊域における沈み込みに関する詳細な構造のイメージングおよび地震のアスペリティに関する構造を明らかにするためのデータの取得、および西南日本の付加体先端部付近で発生している低周波地震や微動を含む自然地震観測を実施した。 　本講演では、四国沖?紀伊半島沖の平成21~22年度の調査結果について述べる。平成21年10月、および平成22年10~11月、（独）海洋研究開発機構の海洋調査船「かいれい」によって短周期海底地震計各々180台と大容量チューンドエアガン（7800cu. in.)を用いた屈折法・広角反射法探査を実施した（図１）。海底地震計設置期間中に自然地震観測も実施した。四国沖では21観測点、紀伊半島沖では20観測点による約9ヶ月間の長期地震観測も実施した（一部実施中）。なお、本調査は文部科学省からの受託研究「東海・東南海・南海地震の連動性評価のための 調査観測・研究」の個別研究テーマ「南海トラフ域海域地震探査・地震観測」（平成20年度から受託）の一環として実施した。 　一部の調査測線の解析の結果、足摺岬沖から日向灘に向かって約6km/sの古い付加体を示す岩体の分布が海側に張り出していること、また、SK05の構造モデルによると、SK03とSK02の中間付近からSK01付近までの付加堆積物が極端に薄いことなど、トラフ平行方向に構造変化があることがわかり、破壊様式の違いに関係する構造ではないかと考えられる。また、測線延長上のHi-net陸上観測点のデータを加えた海陸統合解析を実施しており、これにより深部低周波地震現象と構造との関連性が明らかになると期待されるC11-10発表要旨, 日本地震学会2011年度秋季大会（2011年10月12日～15日, 静岡県静岡市

Prominent reflector beneath around the segmentation boundary between Tonankai-Nankai earthquake area

In the Nankai Trough subduction seismogenic zone, the Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. In most cases, first break of such large events of Nankai Trough usually begins from southwest off the Kii Peninsula so far. The idea of split Philippine Sea plate between the Kii Peninsula and the Shikoku Island, which explains seismicity, tectonic background, receiver function image and historical plate motion, was previously suggested. Moreover, between the Kii Peninsula and the Shikoku Island, there is a gap of deep low-frequency events observed in the belt-like zone along the strike of the subducting Philippine Sea plate. In 2010 and 2011, we conducted the large-scale high-resolution wide-angle and reflection (MCS) seismic study, and long-term observation from off Shikoku and Kii Peninsula. Marine active source seismic data have been acquired along grid two-dimensional profiles having the total length of ~800km/year. A three-dimensional seismic tomography using active and passive seismic data observed both land and ocean bottom stations have been also performed. From those data, we found a possible prominent reflector imaged in the offshore side in the Kii channel at the depth of ~18km. The velocity just beneath the reflector cannot be determined due to the lack of ray paths. Based of the amplitude information, we interpret the reflector as the forearc Moho based on the velocity gap (from ~6.4km/s to ~7.4km/s). However, the reflector is shallower than the forearc Moho of other area along the Nankai Trough. Similar reflectors are recognized along other seismic profiles around the Kii channel. In this presentation, we will show the result of structure analysis to understand the peculiar structure including the prominent reflector around the Kii channel. Relation between the structure and the existence of the segmentation of the Nankai megathrust earthquake or seismic gap of the deep low-frequency events will be also discussed. This study is part of 'Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan.Poster abstract T43C-2670 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec

地震発生帯における深部掘削孔を用いた長期計測

Large earthquakes occur frequently in subduction zones. Most earthquakes are generated in the seismogenic zone, a fairly limited area confined to the shallower regions of the subduction plate boundary. To understand the processes of earthquake generation, it is essential to monitor the physical and mechanical properties of the seismogenic zone over long periods. At present, there are no deep borehole observations of the seismogenic zone more than 3km below seafloor, because it has, until now, been impossible to penetrate to such depths below the sea floor. The Integrated Ocean Drilling Program (IODP), scheduled to begin in 2003, plans to drill boreholes beneath the ocean floor using a multiple-drilling platform operation. The IODP riser-quipped drilling ship (Chikyu) enables the emplacement of boreholes up to 0km beneath the ocean floor, and will provide opportunities to conduct long-term deep borehole observations in the seismogenic zone. Long-term borehole observations in the seismogenic zone are expected to require the development of advanced sampling, monitoring, and recording technology. Here, we discuss the scientific objectives, engineering and technical challenges, and experimental design for a deep borehole, long-term deepborehole monitoring system aimed at understanding the processes of earthquake generation in the seismogenic zone of subduction plate boundaries. We focus specifically on the relationships between environmental conditions in the deep subsurface, details of monitoring and recording, and design and implementation of scientific tools and programs